A magnetorheological fluid (MRF) is a fluid that responds to a magnetic field by changing its rheological behavior. Typically, such a fluid will reversibly change from a free-flowing, linear viscous liquid to a semi-solid when exposed to a magnetic field of sufficient strength. In practice, the time interval over which this change takes place is often limited by the characteristics of the field coil that produces the applied magnetic field, and of the circuitry that drives the field coil. Typical MRFs are particulate suspensions of iron or magnetic alloy in a carrier liquid which may, e.g., be petroleum-based, or water- or glycol-based.
The yield strength of a MRF is magnetically controllable. This property can be utilized in devices that include a magnetic field source, and that contain a MRF. The MRF can be made selectively resistant to flow or shear in portions of the device that are subjected to the magnetic field.
Such devices have been described. For example, U.S. Pat. No. 5,277,281, which issued on Jan. 11, 1994 to J. D. Carlson et al., discloses a fluid damper that utilizes the properties of a MRF. Briefly, a piston moves up and down within a MRF-filled chamber. The piston effectively divides the chamber into upper and lower portions interconnected by a narrow annular channel between the piston and the inner wall of the chamber. When the piston is displaced, MRF is required to flow through this channel. A field coil is included within the piston. When the field coil is energized, a magnetic field permeates the channel and excites a transformation of the MRF to a state that exhibits substantial damping forces.
Devices of this kind have proven to be quite useful. However, they suffer from certain limitations that tend to restrict their range of applications. For example, there is a need for MRF devices that can be switched more rapidly than those that are generally available at present. However, for a given flux density required to control the MRF, the response time of the device tends to go up as the amount of energy stored in the magnetic field is increased. Generally, this quantity tends to increase when steel (or other ferrous metal) components are made part of the magnetic circuit; i.e., when such components are traversed by the magnetic field lines that act upon the MRF. In conventional MRF devices, the magnetic field lines generally traverse a substantial volume of steel. As a consequence, the degree to which the response time of these devices can be shortened is limited.
Furthermore, there is a need for a compact, powerful MRF device such as one that can generate dynamic forces on the order of 50 tons within a package having a maximum dimension on the order of 10 inches or less. Such a device can be achieved only by increasing the magnetic flux density, within the MRF, beyond the levels that are typically encountered in conventional MRF devices. A compact, powerful MRF device will be especially useful if it can be switched between the damping and non-damping states within a relatively short time interval, such as an interval of a few milliseconds or less.
However, there inheres in conventional MRF devices a conflict between the goal of increasing magnetic flux density, and the goal of preserving short response times. As noted, the response time tends to go up as the amount of energy stored in the magnetic field is increased, particularly when there is steel (or other saturable materials) in the magnetic circuit. This effect may become greatly magnified at high values of the flux density. Under such conditions, the required field strength from the coil may be sufficient to cause magnetic saturation of the steel components. As saturation is approached, there is a steep increase in the field strength needed to produce a given flux density and therefore a steep increase in the stored energy. The previous referenced U.S. Pat. No. 5,277,281, discusses the design limitations to which conventional MRF devices are subject.
Another limitation of conventional MRF devices relates to the seal between the high-pressure side of the device and the environment (which is typically the atmosphere). Such a seal is typically a sliding seal that admits, or a deformable seal that seals around, a moveable piston rod. The resistance of the MRF to motions of the piston rod generates high pressures within the device. Unusually high pressures, such as may be caused by unexpectedly severe shock loads, may result in seal failure.
Thus, there has remained a need for MRF devices that can be operated with higher magnetic flux densities, while remaining limited in spatial dimensions and preserving short response times. There has also remained a need for MRF devices that are capable of withstanding exceptionally high resistive forces.